2,060 research outputs found
Destabilization of rotating flows with positive shear by azimuthal magnetic fields
According to Rayleigh's criterion, rotating flows are linearly stable when
their specific angular momentum increases radially outward. The celebrated
magnetorotational instability opens a way to destabilize those flows, as long
as the angular velocity is decreasing outward. Using a short-wavelength
approximation we demonstrate that even flows with very steep positive shear can
be destabilized by azimuthal magnetic fields which are current-free within the
fluid. We illustrate the transition of this instability to a rotationally
enhanced kink-type instability in case of a homogeneous current in the fluid,
and discuss the prospects for observing it in a magnetized Taylor-Couette flow.Comment: 4 pages, 4 figur
Standard and helical magnetorotational instability: How singularities create paradoxal phenomena in MHD
The magnetorotational instability (MRI) triggers turbulence and enables
outward transport of angular momentum in hydrodynamically stable rotating shear
flows, e.g., in accretion disks. What laws of differential rotation are
susceptible to the destabilization by axial, azimuthal, or helical magnetic
field? The answer to this question, which is vital for astrophysical and
experimental applications, inevitably leads to the study of spectral and
geometrical singularities on the instability threshold. The singularities
provide a connection between seemingly discontinuous stability criteria and
thus explain several paradoxes in the theory of MRI that were poorly understood
since the 1950s.Comment: 25 pages, 10 figures. A tutorial paper. Invited talk at SPT 2011,
Symmetry and Perturbation Theory, 5 - 12 June 2011, Otranto near Lecce
(Italy
Extending the range of the inductionless magnetorotational instability
The magnetorotational instability (MRI) can destabilize hydrodynamically
stable rotational flows, thereby allowing angular momentum transport in
accretion disks. A notorious problem for MRI is its questionable applicability
in regions with low magnetic Prandtl number, as they are typical for
protoplanetary disks and the outer parts of accretion disks around black holes.
Using the WKB method, we extend the range of applicability of MRI by showing
that the inductionless versions of MRI, such as the helical MRI and the
azimuthal MRI, can easily destabilize Keplerian profiles ~ 1/r^(3/2) if the
radial profile of the azimuthal magnetic field is only slightly modified from
the current-free profile ~ 1/r. This way we further show how the formerly known
lower Liu limit of the critical Rossby number, Ro=-0.828, connects naturally
with the upper Liu limit, Ro=+4.828.Comment: Growth rates added, references modified; submitted to Physical Review
Letter
Determining role of Krein signature for 3D Arnold tongues of oscillatory dynamos
Using a homotopic family of boundary eigenvalue problems for the mean-field
-dynamo with helical turbulence parameter
and homotopy parameter , we show that the underlying network of diabolical points for Dirichlet
(idealized, ) boundary conditions substantially determines the
choreography of eigenvalues and thus the character of the dynamo instability
for Robin (physically realistic, ) boundary conditions. In the
space the Arnold tongues of oscillatory solutions at
end up at the diabolical points for . In the vicinity of the
diabolical points the space orientation of the 3D tongues, which are cones in
first-order approximation, is determined by the Krein signature of the modes
involved in the diabolical crossings at the apexes of the cones. The Krein
space induced geometry of the resonance zones explains the subtleties in
finding -profiles leading to spectral exceptional points, which are
important ingredients in recent theories of polarity reversals of the
geomagnetic field.Comment: 4 pages, 3 figures, presented at the GAMM 2008, Bremen, Germany
Introduction extended, refs adde
Instabilities of rotational flows in azimuthal magnetic fields of arbitrary radial dependence
Using the WKB approximation we perform a linear stability analysis for a
rotational flow of a viscous and electrically conducting fluid in an external
azimuthal magnetic field that has an arbitrary radial profile B_{phi}(R). In
the inductionless approximation, we find the growth rate of the
three-dimensional perturbation in a closed form and demonstrate in particular
that it can be positive when the velocity profile is Keplerian and the magnetic
field profile is slightly shallower than R^{-1}.Comment: 15 pages, 2 figures, slightly extended, the case of finite Rm
treated, results were partially presented at the IUTAM Symposium on Vortex
Dynamics, Fukuoka, Japan, March 10 201
On the relation of standard and helical magnetorotational instability
The magnetorotational instability (MRI) plays a crucial role for cosmic
structure formation by enabling turbulence in Keplerian disks which would be
otherwise hydrodynamically stable. With particular focus on MRI experiments
with liquid metals, which have small magnetic Prandtl numbers, it has been
shown that the helical version of this instability (HMRI) has a scaling
behaviour that is quite different from that of the standard MRI (SMRI). We
discuss the relation of HMRI to SMRI by exploring various parameter
dependencies. We identify the mechanism of transfer of instability between
modes through a spectral exceptional point that explains both the transition
from a stationary instability (SMRI) to an unstable travelling wave (HMRI) and
the excitation of HMRI in the inductionless limit. For certain parameter
regions we find new islands of the HMRI.Comment: 48 pages, 13 figure
The economic impact of the bacterial blight of soybean under European agroclimatic conditions
The economic impact of the bacterial blight of soybean caused by Pseudomonas syringae pv. glycinea has been investigated in three countries belonging to the European Union: Italy, France and Spain. Weather and growing conditions have been monitored over three years of field experiments (1992-1994) and the data analysed in order to evaluate possible yield losses and in view of the production of pathogen free seed. In Italy and France, using different cultivars and seed with a contamination level of 0.5-20% no significant yield losses were found. In Italy the initial seed contamination level was positively correlated with the contamination of the harvested seed by the pathogen; both in Italy and in France on some cultivars, it was possible to correlate seed contamination level with the epiphytic population of the pathogen and the intensity of symptoms affecting plants in the field. No epiphytic contamination by the pathogen was observed in Spain, even at the highest seed contamination rate (20%), and there was no disease in the field and no yield reduction. The pathogen seemed not to become systemic since no contamination was observed on seed aseptically taken in the field just before harvest. The experiments highlighted the low impact of soybean bacterial blight under European climatic conditions, but suggested the choice of dry and warm regions for the production of quality seed to prevent the accumulation of effective inoculum on the seed year by year
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